The study was aimed at applying molecular marker techniques to locate QTL and determine the efficiency of the marker-assisted selection. The research was done using Brassica oleracea and Arabidopsis thaliana. The Brassica DH lines represented a population of homozygous individuals while the F\(_2\) and F\(_3\) generations of Arabidopsis represented a segregating population. Marker-assisted selection was applied after the detection of QTL which allowed the identification of markers linked to the QTL and hence the selection for such markers. In Brassica, 40 QTL were detected using the marker regression method. Between 1 and 6 QTL were located per trait, which individually explained 2-49% of the additive genetic variance. In Arabidopsis the marker regression method detected 23 QTL in the F\(_2\), whereas 40 QTL were detected by the interval mapping method in the F\(_3\) generation. 1 7 QTL mapping to similar positions and showing similar modes of action were detected by both methods. Alleles for various QTL were dispersed between parents in both crosses. The efficiency of MAS was determined using various approaches, based on the number of top ranks, number of lines in a group, phenotypic value and as the ratio between response based on MAS and response obtained in the F\(_3\) by applying phenotypic selection to the F\(_2\) generation. The MAS gave generally better response compared to phenotypic selection, particularly when heritability was low. MAS for single QTL was always more effective while multiple QTL and QTL showing linkage posed some practical problems in MAS applications. Overall, MAS has to be applied in conjunction with phenotypic selection to get best results as QTL of minor effect cannot be tackled through marker/QTL associations.